The present invention relates to a light source-optical fiber coupler having a gradient index rod lens for coupling light emitted from a light source to an end surface of an optical fiber. More specifically, it relates to a light source-optical fiber coupler which has a gradient index rod lens having a light source side end surface shaped like a convex spherical surface, and an optical fiber side end surface shaped like a flat surface, and in which the light source and the gradient index rod lens are disposed close to each other to thereby make reduction in size possible.
In optical communication or the like, diffuse luminous flux emitted from a light source such as a semiconductor laser is transmitted to a core of an optical fiber by a lens. Examples of the lens heretofore used for such optical coupling are a spherical lens, an aspherical lens, a gradient index rod lens, and so on.
Although the spherical lens can be produced most inexpensively, the spherical lens is insufficient to optically couple a semiconductor laser to a single mode optical fiber with a small loss. This is because the single mode optical fiber has such a small core diameter that very small lens aberration is required for providing high optical coupling efficiency. Therefore, at present, an aspherical lens is generally used for high coupling efficiency. Therefore, at present, an aspherical lens is generally used as a high coupling lens.
On the other hand, when a gradient index rod lens is used, at least one end surface is often processed into a spherical surface because a high numerical aperture (NA) is required for coupling light emitted from a semiconductor laser. This is based on the thought that condensing power is obtained by a convex spherical surface and that aberration generated in the convex spherical surface is cancelled by the refractive-index distribution of the gradient index rod lens.
FIG. 6 shows an example of the background art. A semiconductor laser used as a light source has a package mount structure (such as a TO type structure) in which a laser chip 10 is covered with a cap having a window portion so that laser light is emitted while transmitted through cover glass 12 in the window portion. The light emitted from the laser is focused onto an end surface of a single mode optical fiber 16 by a gradient index rod lens 14. In this example, the gradient index rod lens 14 is assembled so that a semiconductor laser side end surface 14a is shaped like a convex spherical surface and an optical fiber side end surface 14b is shaped like a flat surface.
For production of an aspherical lens, however, a mold is required in accordance with the kind of the lens. A heat-resistant material and a super-precision process are required for the production. Accordingly, particularly in the case of large item small volume production, there is a problem that the cost of the lens becomes high.
On the contrary, a gradient index rod lens has an advantage in that the lens is small in size and can be produced inexpensively and easily. On the other hand, since a semiconductor laser is easy to handle at the time of assembling, a semiconductor laser having a structure to be mounted in a package is often used. In this case, however, the distance between a laser chip surface (emission position) and an end surface of the gradient index rod lens cannot be made shorter than a specific value because of the presence of cover glass 12. The shortest distance is generally in a range of from about 0.6 to about 0.7 mm. For this reason, coupling loss is increased because of eclipse of light emitted from the semiconductor laser, so that it is difficult to couple the semiconductor laser to the single mode optical fiber efficiently.